[iSCSI] Fix incorrect heuristic.

The only valid check for Expected Data Transfer Length is :
R and W	     MUST  NOT both be 0 when the Expected Data Transfer Length and/or
Bidirectional Read Expected Data Transfer Length are not 0

Backported with manual intervention from I5503a5ad524c4e237b143696fc8eda895bcb9172
with change to reflect v1.12 version of packet-isci.c code.

Ping-Bug: 10469

Change-Id: I4b3ed968a092ff3a6704f94f4517d45793a599cf
Reviewed-on: https://code.wireshark.org/review/4530
Reviewed-by: Michael Mann <mmann78@netscape.net>